Electrooptic modulation element

ABSTRACT

On one pair of opposed side faces ( 1   a,    1   b ) of electro-optic crystal ( 1 ), grooves ( 3   a,    3   b ) are formed so as to make bottom faces of the grooves approach each other and make a distance between the bottom faces shorter than a predetermined distance, and the pair of electrodes ( 5   a,    5   b ) are formed in the grooves ( 3   a,    3   b ) so as to fill the grooves nearly completely.

This application is a national phase application under 35 U.S.C. §371 ofInternational Application No. PCT/JP04/08384, filed Jun. 9, 2004, whichclaims priority to JP 2003-165497, filed in Japan on Jun. 10, 2003, JP2003-380434, filed in Japan on Nov. 10, 2003, and JP 2004-111861, filedin Japan on Apr. 6, 2004.

TECHNICAL FIELD

The present invention relates to an electro-optic modulation device thatincludes electro-optic (EO) crystal having an variable birefringenceindex according to a coupled electric field, and one pair of electrodesdisposed so as to have the electro-optic crystal interposed therebetweento couple the electric field to the electro-optic crystal, and thatchanges polarization of light incident between the one pair ofelectrodes according to a change of the birefringence index dependingupon a strength of electric field coupled via the one pair ofelectrodes. In particular, the present invention relates to anelectro-optic modulation device improved in modulation efficiency andsensitivity and flattened in frequency characteristics without hamperingthe strength and increasing the size.

BACKGROUND ART

An electro-optic modulation device using electro-optic crystal is usedas an electro-optic modulator which modulates the phase of light passedthrough the crystal according to the magnitude of the electric fieldgenerated between electrodes, or as an electric-field sensor forconversely detecting a phase change of light passed through the crystaland thereby detecting the electric field between the electrodes or anelectric signal.

For example, in the electric-field sensor, an optical beam is incidenton electro-optic crystal with AC electric field applied thereto andlight emitted from the electro-optic crystal is separated intoS-polarized light and P-polarized light by a polarizing-beam splitter(hereafter referred to as PBS). The polarized lights are detectedrespectively and independently by two photodetectors (hereafter referredto as PD), and a difference between intensities of the S-polarized lightand the P-polarized light is detected by the PD and a differentialamplifier.

FIG. 1 is a diagram showing operation of a conventional electric-fieldsensor.

An optical beam emitted from a light source 101 is transmitted through aphase compensator 105 and electro-optic crystal 107, and then incidenton a PBS 109. The polarization state of the optical beam 103 is adjustedby the phase compensator 105 so as to become circularly polarized lightimmediately before incidence on the PBS 109. An electric field dependingon a signal 115 to be measured is applied to the electro-optic crystal107 via a signal electrode 111 and a ground electrode 113. The opticalbeam 103 is subjected to polarization modulation in the electro-opticcrystal 107 according to the electric field. The polarized modulatedlight is separated into an S-polarized component and a P-polarizedcomponent by the PBS 109. At this time, each polarized component hasalready been converted to intensity modulated light. The intensitymodulated S-polarized component and P-polarized light change in phasesopposite to each other. Accordingly, by receiving light in PDs 117 and119 and conducting differential signal detection in a differentialamplifier 121, therefore, it becomes possible to obtain an output signal122 having a higher signal-to-noise ratio (see, for example, JapanesePatent Application Laid-Open Nos. 2003-98205, 2003-98204 and2000-171488).

The electro-optic modulation device using electro-optic crystal beginsto be applied to communication between wearable computers using a livingbody as a signal path. In other words, by inducing electric field in areceiver in a wearable computer of communication destination via theliving body and detecting the electric field by using an electro-optictechnique, communication that does not depend upon the positionalrelation between the ground of the wearable computer and the earthground to the utmost, that is communication with a wearable computerthat is in an arbitrary position on the living body, can be certainlyimplemented.

FIGS. 2A to 2C are diagrams to explain a process for fabricating anelectro-optic modulation device by using electro-optic crystal.

An electro-optic modulation device including electro-optic crystal and apair of electrodes is formed by working thinly electro-optic crystal 107a of a raw material as shown in FIG. 2A to form a thin electro-opticcrystal 107 as shown in FIG. 2B, and forming a pair of electrodes 111and 113 on a pair of opposite side faces of the electro-optic crystal107 worked to become thin. By the way, an electro-optic crystal 101 athus worked to become thin has a thickness d of approximately 0.1 mm.

As communication using a living body as the transmission path, severalmodes are conceivable. As representative modes, two modes such ascommunication between an installation type terminal and a portableterminal, and communication between portable terminals are conceivable.

In the communication between an installation type terminal and aportable terminal, communication can be conducted in a comparativelystable state since the installation type is connected to the earthground. On the other hand, in the communication between portableterminals, communication is conducted in an extremely unstable statesince neither of the terminals is grounded. Furthermore, battery driveis conducted typically and low power consumption is demanded. Therefore,conditions imposed on a receiver to establish communication in such astate are high sensitivity and flatness in the frequency characteristicsof the sensitivity.

First studying the sensitivity, a phase change (Δφ) given to light bythe electro-optic modulation device is given by the followingexpression.Δφ=α·(V/d)·L

Here, α is a constant depending upon the kind of the electro-opticcrystal and the structure of the device, V is a voltage applied to theelectrodes, d is a distance between the electrodes, and L is a length ofthe electro-optic modulation device. As represented by the expression, agreater phase change can be given to light as d becomes small and Lbecomes large. In other words, the modulation efficiency becomes high asan electro-optic modulator, and the sensitivity is improved as anelectric-field sensor.

In order to shorten the distance between the pair of electrodes to theutmost, it is necessary to work the electro-optic crystal to make itthin. In the conventional technique, however, there is a problem that itis extremely difficult to generate a device using thin crystal having athickness of mm order or less and the electro-optic crystal becomes aptto break.

It is desirable to apply antireflection coating to an end face of theelectro-optic crystal on which light is incident. However, there is aproblem that it becomes difficult to apply the antireflection coating ifthe electro-optic crystal is made thin.

From a different point of view, the sensitivity of the electro-opticmodulation device serving as an electric-field sensor can be improved bylengthening the length of the electro-optic crystal in a light passagedirection as described above. If the electro-optic modulation device isprovided with a specific structure in order to increase the intensity bymaking the electro-optic crystal thin, then a phenomenon that light doesnot emit from the end face of the electro-optic crystal and light leaksin a side face direction because of light diffraction as the length ismade longer is caused, resulting in a lowered modulation efficiency or alowered sensitivity.

As for the flatness of the frequency characteristics which is the seconddemand, the following fact poses a problem. That is, in theelectro-optic crystal with electric field applied thereto, thebirefringence index of the crystal with respect to light is changed bydeformation of the electron cloud and crystal lattice. The degree of thedeformation of the electron cloud does not depend on the frequency ofthe applied electric field, however, the degree of the deformation ofthe crystal lattice depends upon the frequency. In the band of kHz toMHz order, therefore, the frequency characteristics of the sensitivityof the electro-optic crystal do not become flat in general. The reasonwhy the frequency characteristics of the electro-optic crystal do notbecome flat is specifically that the eigenmode of the elastic vibrationis caused depending upon the size and shape of the crystal.

In view of these problems, the present invention has been achieved. Anobject of the present invention is to provide an electro-opticmodulation device capable of improving the modulation efficiency andsensitivity.

In particular, an object of the present invention is to provide anelectro-optic modulation device capable of improving the modulationefficiency and sensitivity without hampering the strength of the deviceand causing a leak of light due to diffraction even when the gap betweenthe pair of electrodes is made narrow.

Further, more specifically, an object of the invention is to provide anelectro-optic modulation device which has flatness in the frequencycharacteristic.

DISCLOSURE OF THE INVENTION

In order to achieve the objects, a spirit of invention according to afirst aspect is an electro-optic modulation device that includeselectro-optic crystal having a birefringence index changed by a coupledelectric field, and one pair of electrodes disposed so as to have theelectro-optic crystal interposed therebetween to couple the electricfield to the electro-optic crystal, and that changes polarization oflight incident between the one pair of electrodes according to a changeof the birefringence index depending upon a strength of electric fieldcoupled via the one pair of electrodes, wherein the electro-opticcrystal includes grooves parallel to a direction of the incident lightrespectively on one pair of side faces parallel to the direction, andconsequently a thin crystal portion sandwiched between the groovesserves as a portion for coupling the electric field, and the one pair ofelectrodes are formed so as to fill the grooves, respectively.

In accordance with a spirit of invention according to a second aspect,the grooves are formed on the one pair of side faces so as to range fromone to the other of end faces through which light is incident oremitted, in the invention according to the first aspect.

In accordance with a spirit of invention according to a third aspect,the grooves are formed in only a central portion except end portionsbetween the end faces through which light is incident or emitted, in theone pair of side faces, in the invention according to the first aspect.

In order to achieve the objects, a spirit of invention according to afourth aspect is an electro-optic modulation device that includeselectro-optic crystal having a birefringence index changed by a coupledelectric field, and one pair of electrodes disposed so as to have theelectro-optic crystal interposed therebetween to couple the electricfield to the electro-optic crystal, and that changes polarization oflight incident between the one pair of electrodes according to a changeof the birefringence index depending upon a strength of electric fieldcoupled via the one pair of electrodes, wherein the electro-opticcrystal includes grooves parallel to a direction of the incident lightrespectively on one pair of side faces parallel to the direction, andconsequently a thin crystal portion sandwiched between the groovesserves as a portion for coupling the electric field, the one pair ofelectrodes are formed in bottom portions of the grooves so as to have apredetermined thickness, and at least remaining portions of the groovesexcept the one pair of electrode portions are filled with insulators.

In accordance with a spirit of invention according to a fifth aspect,the grooves are formed on the one pair of side faces so as to range fromone to the other of end faces through which light is incident oremitted, in the invention according to the fourth aspect.

In accordance with a spirit of invention according to a sixth aspect,the grooves are formed in only a central portion except end portionsbetween the end faces through which light is incident or emitted, in theone pair of side faces, in the invention according to the fourth aspect.

In accordance with a spirit of invention according to a seventh aspect,remaining portions of the grooves except the one pair of electrodeportions are filled with insulators, and a whole of portions except theend faces through which light is incident or emitted is covered byfurther insulators, in the invention according to the fourth to sixthaspects.

In accordance with a spirit of invention according to an eighth aspect,the insulators are wax, in the invention according to the fourth toseventh aspects.

In order to achieve the objects, a spirit of invention according to aninth aspect is an electro-optic modulation device that includeselectro-optic crystal having a birefringence index changed by a coupledelectric field, and one pair of electrodes disposed so as to have theelectro-optic crystal interposed therebetween to couple the electricfield to the electro-optic crystal, and that changes polarization oflight incident between the one pair of electrodes according to a changeof the birefringence index depending upon a strength of electric fieldcoupled via the one pair of electrodes, the electro-optic modulationdevice including a base portion, and a ridge-shaped ridge portionprojected on one side face of the base portion and extended in adirection of the incident light, at least a part of the ridge portionincluding the electro-optic crystal, the ridge portion having a widthequivalent to a predetermined value or less, wherein the one pair ofelectrodes are formed on one pair of side faces opposed in a widthdirection of the ridge portion.

In accordance with a spirit of invention according to a tenth aspect,the ridge portion is formed nearly in the center on the one side face ofthe base portion when seen from the direction of the light incidence, inthe invention according to the ninth aspect.

In accordance with a spirit of invention according to an eleventhaspect, the ridge portion is formed on an end on the one side face ofthe base portion when seen from the direction of the light incidence, inthe invention according to the ninth aspect.

In accordance with a spirit of invention according to a twelfth aspect,the electro-optic modulation device further includes an insulator whichcovers the whole, in the invention according to the ninth aspect.

In accordance with a spirit of invention according to a thirteenthaspect, the electro-optic modulation device further includes aninsulator which covers the ridge portion, in the invention according tothe ninth aspect.

In accordance with a spirit of invention according to a fourteenthaspect, the electro-optic modulation device further includes aninsulator which covers a top surface of the ridge portion and side facesof the one pair of electrodes forming faces continuous to the topsurface, in the invention according to the ninth aspect.

In accordance with a spirit of invention according to a fifteenthaspect, the insulator includes wax, in the invention according to thetwelfth to fourteenth aspects.

In accordance with a spirit of invention according to a sixteenthaspect, the electro-optic modulation device includes a low refractiveindex medium having a refractive index which is lower than a refractiveindex of the electro-optic crystal, at least near a side face of theridge portion located on the base side and included in one pair of sidefaces other than the one pair of side faces on which the one pair ofelectrodes are formed, in the invention according to the ninth aspect.

In accordance with a spirit of invention according to a seventeenthaspect, the ridge portion includes the electro-optic crystal, and thebase portion includes the low refractive index medium, in the inventionaccording to the sixteenth aspect.

In accordance with a spirit of invention according to a eighteenthaspect, the ridge portion and an upper part of the base portion includethe electro-optic crystal, and a remaining lower part of the baseportion includes the low refractive index medium, in the inventionaccording to the sixteenth aspect.

In accordance with a spirit of invention according to a nineteenthaspect, the base portion and a lower part of the ridge portion includethe low refractive index medium, and a remaining upper part of the ridgeportion includes the electro-optic crystal, in the invention accordingto the sixteenth aspect.

In accordance with a spirit of invention according to a twentiethaspect, the low refractive index medium is electro-optic crystal whichincludes chemical elements of the same kinds as those of theelectro-optic crystal, but which is lower in refractive index on thebasis of a difference in composition ratio, in the invention accordingto the seventeenth to nineteenth aspects.

In accordance with a spirit of invention according to a twenty-firstaspect, the ridge portion includes the electro-optic crystal, an upperpart of the base portion includes an adhesive agent, and a remaininglower part of the base portion includes a substrate, in the inventionaccording to the sixteenth aspect.

In accordance with a spirit of invention according to a twenty-secondaspect, the ridge portion and an upper part of the base portion includethe electro-optic crystal, a lower part of the electro-optic crystal ofthe base portion includes an adhesive agent, and a remaining lower partof the base portion includes a substrate, in the invention according tothe sixteenth aspect.

In accordance with a spirit of invention according to a twenty-thirdaspect, the base portion includes a substrate, a lower part of the ridgeportion includes an adhesive agent, and a remaining upper part of theridge portion includes the electro-optic crystal, in the inventionaccording to the sixteenth aspect.

In accordance with a spirit of invention according to a twenty-fourthaspect, the low refractive index medium includes gas or a vacuum statein a cavity provided in an upper part of the base portion, in theinvention according to the sixteenth aspect.

In accordance with a spirit of invention according to a twenty-fifthaspect, the ridge portion includes the electro-optic crystal, and thebase portion includes photonic crystal having a periodic structure, inthe invention according to the ninth aspect.

In order to achieve the objects, a spirit of invention according to atwenty-sixth aspect is an electro-optic modulation device that includeselectro-optic crystal having a birefringence index changed by a coupledelectric field, and one pair of electrodes disposed so as to have theelectro-optic crystal interposed therebetween to couple the electricfield to the electro-optic crystal, and that changes polarization oflight incident between the one pair of electrodes according to a changeof the birefringence index depending upon a strength of electric fieldcoupled via the one pair of electrodes, the electro-optic modulationdevice further including an insulator applied so as to relatively fixthe electro-optic crystal and the one pair of electrodes, except endfaces through which light is incident or emitted.

In accordance with a spirit of invention according to a twenty-seventhaspect, the insulator includes a matter that has viscosity and aproperty of becoming hard with the lapse of time, in the inventionaccording to the twenty-sixth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram explaining operation of a conventionalelectric-field sensor.

FIGS. 2A to 2C are diagrams for explain a process for fabricating anelectro-optic modulation device by using electro-optic crystal.

FIGS. 3A and 3B are respectively a partial perspective oblique view anda sectional view showing an electro-optic modulation device according toan embodiment of the present invention.

FIGS. 4A to 4D are diagrams showing a manufacture process of anelectro-optic modulation device having a configuration shown in FIG. 3A.

FIGS. 5A and 5B are respectively a partial perspective oblique view anda sectional view showing an electro-optic modulation device according toanother embodiment of the present invention.

FIGS. 6A to 6D are diagrams showing a manufacture process of anelectro-optic modulation device having a configuration shown in FIG. 5A.

FIGS. 7A and 7B are respectively a partial perspective oblique view anda sectional view showing an electro-optic modulation device according toanother embodiment of the present invention.

FIGS. 8A to 8D are diagrams showing a manufacture process of anelectro-optic modulation device having a configuration shown in FIG. 7A.

FIGS. 9A to 9C are a longitudinal sectional view of a structure of anelectro-optic modulation device according to the embodiment shown inFIGS. 7A and 7B seen from a direction perpendicular to FIG. 7B, andlongitudinal sectional views showing other structures of cornerportions.

FIGS. 10A to 10D are sectional views showing a manufacture process of anelectro-optic modulation device according to another embodiment of thepresent invention.

FIG. 11 is a diagram showing an electro-optic modulation device shown inFIG. 10D with electrodes formed of unnecessary metal remained onelectro-optic crystal being removed.

FIGS. 12A to 12D are sectional views showing a manufacture process of anelectro-optic modulation device according to another embodiment of thepresent invention.

FIG. 13 is a diagram showing a plane of light incidence of anelectro-optic modulation device according to an embodiment of ridgetype.

FIGS. 14A and 14B are diagrams showing a ridge electro-optic modulationdevice using photonic crystal.

FIG. 15 is a diagram showing a plane of light incidence of anelectro-optic modulation device according to another embodiment of ridgetype.

FIG. 16 is a diagram showing a plane of light incidence of anelectro-optic modulation device according to another embodiment of ridgetype.

FIGS. 17A and 17B are diagrams showing a plane of light incidence of anelectro-optic modulation device according to another embodiment of ridgetype.

FIGS. 18A to 18E are diagrams showing how wax is applied toelectro-optic crystal placed longitudinally on a pedestal.

FIG. 19 is a diagram showing differences in output characteristics of anelectric-field sensor among the case where wax is not applied toelectro-optic crystal, the case where wax is applied to a top surface ofthe electro-optic crystal, and the case where wax is applied from thetop surface of the electro-optic crystal to both electrodes, and furtherto a pedestal.

FIGS. 20A to 20E are diagrams showing how wax is applied toelectro-optic crystal placed laterally on a pedestal.

FIGS. 21A and 21B are diagrams showing how wax is applied to anelectro-optic modulation device of the so-called H-type.

FIGS. 22A to 22C are diagrams showing how wax is applied to anelectro-optic modulation device of the so-called ridge type.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereafter, embodiments of the present invention will be described withreference to the drawings.

FIGS. 3A and 3B are respectively a partial perspective oblique view anda sectional view showing an electro-optic modulation device according toan embodiment of the present invention.

The electro-optic modulation device according to the embodiment shown inFIGS. 3A and 3B includes electro-optic crystal 1 having a pair ofgrooves dug and formed in a side face 1 a and a side face 1 b oppositeto the side face 1 a in their longitudinal direction so as to extendfrom an end face 1 c to an end face 1 d, and a pair of electrodes 5 aand 5 b formed of metal embedded in the pair of grooves. By the way, asshown in FIG. 3B, each of the pair of grooves has a rectangularsectional shape, and each of the pair of electrodes 5 a and 5 b buriesits groove nearly completely. In other words, as shown in FIG. 3B, theelectro-optic modulation device according to the present embodiment hasa rectangular sectional shape, when the electro-optic crystal 1 and thepair of electrodes 5 a and 5 b are seen collectively. Furthermore, theelectro-optic modulation device according to the present embodiment issometimes called “H-type” on the basis of the sectional shape of theelectro-optic crystal 1.

By the way, the pair of grooves are formed by, for example, digging boththe side faces 1 a and 1 b by means of cutting or polishing so as tomake bottoms approach each other in order to make a distance d betweenthe pair of electrodes 5 a and 5 b equal to a predetermined distance orless. As for dimensions of the electro-optic modulation device thusformed, for example, the distance d between the electrodes 5 a and 5 bis 0.1 mm or less, the length L is approximately 2 cm, and dimensions tand x of the section respectively in the longitudinal and lateraldirections are approximately 1 cm or less.

Even if the electro-optic modulation device having such a configurationis formed so as to have an extremely small distance d between the pairof electrodes 5 a and 5 b, the electrodes 5 a and 5 b are formed so asto nearly completely embed the grooves formed in the electro-opticcrystal 1 as a whole and the thin crystal portion between the electrodes5 a and 5 b is formed so as to be generally covered by the electrodes 5a and 5 b and the electro-optic crystal 1. Therefore, the electro-opticcrystal 1 is not easily broken from the thin portion between theelectrodes 5 a and 5 b. In addition, the thin structure between theelectrodes 5 a and 5 b is also formed by cutting or polishing theelectro-optic crystal 1 of the raw material from both side faces 1 a and1 b. Therefore, it is not difficult to conduct working so as to make theportion between the electrodes 5 a and 5 b extremely thin, for example,0.1 mm or less.

As shown in FIG. 3B, a spot beam 123 is incident from an end face of theelectro-optic crystal 1 between the electrodes 5 a and 5 b. As forantireflection coating on the plane of incidence as well, it can beconducted extremely easily and certainly by applying the antireflectioncoating to not only the end face of the thin crystal portion between theelectrodes 5 a and 5 b, but also on an end face 1 c of the wholerectangular electro-optic modulation device including the end face ofthe electro-optic crystal 1 except the thin crystal portion and endfaces of the electrodes 5 a and 5 b.

Furthermore, the electrodes 5 a and 5 b, and the thin crystal portionbetween the electrodes 5 a and 5 b are fixed by the whole electro-opticcrystal. These results in an effect that distortion of the thin crystalportion is suppressed and the frequency characteristics become flat. Bythe way, in the electro-optic modulation device having such aconfiguration, the direction of an electric-field vector generated bythe electrodes 5 a and 5 b is perpendicular to the opposed planes of theelectrodes 5 a and 5 b.

A manufacture method of the electro-optic modulation device having theconfiguration shown in FIGS. 3A and 3B will now be described withreference to FIGS. 4A to 4D. In this electro-optic modulation device,for example, rectangular electro-optic crystal 1 of the raw materialshown in FIG. 2A is first dug from its both side faces 1 a and 1 b bycutting or polishing to form two grooves 3 a and 3 b as shown in FIG.4A.

Then, metal such as silver paste is thinly applied to the grooves 3 aand 3 b as represented by characters 5 aa and 5 ba in FIG. 4B to formthin electrodes 5 aa and 5 ba. Subsequently, as shown in FIG. 4C, leadwires 53 for applying a voltage are adhered to the electrodes 5 aa and 5ba.

As a result, the electro-optic modulation device is formed. In thisexample, however, the electrodes 5 aa and 5 ba have thin structures. Inorder to implement the electro-optic modulation device as shown in FIG.3A and increase the strength, silver paste is further applied onto theelectrodes 5 aa and 5 ba formed of silver paste to fill the gaps. As aresult, an electro-optic modulation device including the electrodes 5 aand 5 b that are equal in thickness to those shown in FIG. 3A iscompleted.

FIGS. 5A and 5B are respectively a partial perspective oblique view anda sectional view showing an electro-optic modulation device according toanother embodiment of the present invention.

In the electro-optic modulation device according to the embodiment shownin FIGS. 5A and 5B, generally thin electrodes 7 a and 7 b having athickness less than a predetermined thickness that is less than thedepth of grooves are formed on bottoms in the grooves 3 a and 3 b,instead of the electrodes 5 a and 5 b generally formed in the grooves 3a and 3 b in the electro-optic modulation device according to theembodiment shown in FIGS. 3A and 3B, and insulators 9 a and 9 b areformed so as to fill the grooves left above the thin electrodes 7 a and7 b and thereby form the whole electro-optic modulation device as onebody.

Even if the electro-optic modulation device having such a configurationis formed so as to have an extremely short distance between the pair ofelectrodes 7 a and 7 b, the electrodes 7 a and 7 b are formed so as tonearly completely fill the grooves 3 a and 3 b formed in theelectro-optic crystal 1 as a whole in conjunction with the insulators 9a and 9 b and generally cover the thin crystal portion between theelectrodes 7 a and 7 b by the electrodes 7 a and 7 b, the insulators 9 aand 9 b and the electro-optic crystal 1. Therefore, the electro-opticcrystal 1 is not broken easily from the thin portion of the electrodes 7a and 7 b. In addition, since the thin crystal structure between theelectrodes 7 a and 7 b respectively in the grooves 3 a and 3 b is alsoformed by digging the electro-optic crystal 1 of the raw material fromboth side faces 1 a and 1 b by means of cutting or polishing, it is notdifficult to conduct working so as to make the portion between theelectrodes 7 a and 7 b extremely thin, for example, 0.1 mm or less.

As shown in FIG. 5B, a spot beam 123 is incident from an end face of theelectro-optic crystal 1 between the electrodes 7 a and 7 b. As forantireflection coating on the plane of incidence as well, it can beconducted extremely easily and certainly by applying the antireflectioncoating to not only the end face of the thin crystal portion between theelectrodes 7 a and 7 b, but also on an end face 1 c of the wholerectangular electro-optic modulation device including the end face ofthe electro-optic crystal 1 except the thin crystal portion and endfaces of the electrodes 7 a and 7 b, and end faces of the insulators 9 aand 9 b.

Furthermore, the electrodes 7 a and 7 b, and the thin crystal portionbetween the electrodes 7 a and 7 b are fixed by the whole electro-opticcrystal and the insulators 9 a and 9 b. These results in an effect thatdistortion of the thin crystal portion is suppressed and the frequencycharacteristics become flat. By the way, in the electro-optic modulationdevice having such a configuration, the direction of an electric-fieldvector generated by the electrodes 7 a and 7 b is perpendicular to theopposed planes of the electrodes 7 a and 7 b.

A manufacture method of the electro-optic modulation device having theconfiguration shown in FIGS. 5A and 5B will now be described withreference to FIGS. 6A to 6D. In this electro-optic modulation device,for example, rectangular electro-optic crystal 1 of the raw materialshown in FIG. 2A is first dug from its both side faces 1 a and 1 b bycutting or polishing to form two grooves 3 a and 3 b as shown in FIG.6A.

Then, a conductive material such as silver paste is thinly applied tothe grooves 3 a and 3 b to form electrodes 7 a and 7 b as shown in FIG.6B. Subsequently, as shown in FIG. 6C, lead wires 53 for applying avoltage are adhered to the electrodes 7 a and 7 b. By the way, themanufacture process is thus far the same as that shown in FIGS. 4A to4C.

Subsequently, as shown in FIG. 6D, the grooves above the thin electrodes7 a and 7 b having the lead wires 53 adhered thereto are filled with theinsulators 9 a and 9 b, leaving no space. In order to fill the groovesleaving no space, for example, an adhesive agent is suitable as theinsulators 9 a and 9 b.

FIGS. 7A and 7B are respectively a partial perspective oblique view anda sectional view showing an electro-optic modulation device according toanother embodiment of the present invention.

Instead of forming a sandwich structure including a pair of electrodes 7a and 7 b, thin electro-optic crystal between the electrodes 7 a and 7b, and the insulators 9 a and 9 b so as to extend from the end face 1 cto the end face 1 d in the electro-optic modulation device according tothe embodiment shown in FIGS. 5A and 5B, the electro-optic modulationdevice according to the embodiment shown in FIGS. 7A and 7C is formed soas to have such a sandwich structure only in a central portion betweenthe end face 1 c and the end face 1 d except end portions. In otherwords, instead of the grooves 3 a and 3 b in the embodiment shown inFIGS. 5A and 5B, concave portions 4 a and 4 b surrounded by theelectro-optic crystal in their periphery are formed, and the sandwichstructure is formed in the concave portions 4 a and 4 b. Otherstructures and operations are the same as those in the embodiment shownin FIGS. 5A and 5B.

Even if the electro-optic modulation device having such a configurationis formed so as to have an extremely small distance d between pair ofelectrodes 7 aa and 7 ba, the electrodes 7 aa and 7 ba are formed so asto nearly completely embed the concave portions 4 a and 4 b formed inthe electro-optic crystal 1 as a whole in conjunction with insulators 9aa and 9 ba and the thin crystal portion between the electrodes 7 aa and7 ba is formed so as to be generally covered by the electrodes 7 aa and7 ba, the insulators 9 aa and 9 ba, and the electro-optic crystal 1.Therefore, the electro-optic crystal 1 is not easily broken from thethin portion between the electrodes 7 aa and 7 ba. In addition, the thinstructure between the electrodes 7 aa and 7 ba in the concave portions 4a and 4 b is also formed by digging the electro-optic crystal 1 of theraw material from both side faces 1 a and 1 b by means of cutting orpolishing. Therefore, it is not difficult to conduct working so as tomake the distance d between the electrodes 7 aa and 7 ba extremelyshort, for example, 0.1 mm or less.

As shown in FIG. 7B, a spot beam 123 is incident on an end face of theelectro-optic crystal 1 between the electrodes 7 aa and 7 ba from theend face of the electro-optic crystal 1 generally covering its outside.As for antireflection coating on the plane of incidence as well, it canbe conducted extremely easily and certainly because the antireflectioncoating is conducted on the whole end face of the electro-optic crystal1.

Furthermore, the electrodes 7 aa and 7 ba, and the thin crystal portionbetween the electrodes 7 aa and 7 ba are fixed by the wholeelectro-optic crystal and the insulators 9 aa and 9 ba. These results inan effect that distortion of the thin crystal portion is suppressed andthe frequency characteristics become flat. By the way, in theelectro-optic modulation device having such a configuration, thedirection of an electric-field vector generated by the electrodes 7 aaand 7 ba is perpendicular to the opposed planes of the electrodes 7 aaand 5 ba.

A manufacture method of the electro-optic modulation device having theconfiguration shown in FIGS. 7A and 7B will now be described withreference to FIGS. 8A to 8D. Nearly in the same way as the manufacturemethod of the electro-optic modulation device shown in FIGS. 5A and 5Bdescribed with reference to FIGS. 6A to 6D, in this manufacture method,rectangular electro-optic crystal of the raw material is first dug fromits both side faces 1 a and 1 b by cutting or polishing to form twoconcave portions 4 a and 4 b each taking the shape of a groove, as shownin FIG. 8A.

Then, a conductive material such as silver paste is thinly applied tothe concave portions 4 a and 4 b to form electrodes 7 aa and 7 ba asshown in FIG. 8B. Subsequently, as shown in FIG. 8C, lead wires 53 forapplying a voltage are adhered to the electrodes 7 aa and 7 ba.

Subsequently, as shown in FIG. 8D, the concave portions above the thinelectrodes 7 aa and 7 ba having the lead wires 53 adhered thereto arefilled by the insulators 9 aa and 9 ba, leaving no space. In order tofill the grooves leaving no space, for example, an adhesive agent issuitable as the insulators 9 aa and 9 ba. As described above, theelectro-optic modulation device shown in FIGS. 7A and 7B can bemanufactured by using the same manufacture method as that of theelectro-optic modulation device shown in FIGS. 5A and 5B except that thegrooves are replaced by concave portions. In the concave portions 4 aand 4 b formed in the electro-optic crystal, however, it is notnecessary that sides thereof are at right angles, but the sides may beinclined or curved as shown in FIGS. 9B and 9C and as described below.

FIG. 9A is a longitudinal sectional view of the structure of theelectro-optic modulation device according to the embodiment shown inFIGS. 7A and 7B seen from a direction perpendicular to FIG. 7B. As shownin FIG. 9A, all corner portions 11 of bottoms of the concave portions 4a and 4 b having the pair of electrodes 7 aa and 7 ba and the insulators9 aa and 9 ba embedded therein are formed nearly at right angles.

On the other hand, FIGS. 9B and 9C are longitudinal sectional viewsshowing other structures of the corner portions. In the case of FIG. 9B,all corner portions on bottoms in the concave portions 4 aa and 4 ba areformed so as to be inclined at an obtuse angle which is larger than aright angle. In the case of FIG. 9C, all corner portions on bottoms inthe concave portions 4 aa and 4 ba are formed so as to be curved roundwithout being angular. In these cases as well, the concave portions maybe filled with only a conductive material to form electrodes.

In the foregoing embodiments, the case where grooves are formed on bothsides has been described. As a matter of course, however, a groove maybe formed on only one side face.

FIGS. 10A to 10D are sectional views showing a manufacture process of anelectro-optic modulation device according to another embodiment of thepresent invention.

The electro-optic modulation device according to the embodiment shown inFIGS. 10A to 10D finally includes a ridge portion 21 formed on the topsurface of the electro-optic crystal 1 so as to project with apredetermined width d or less, for example, with 0.1 mm or less, and apair of electrodes 25 a and 25 b formed on a pair of side faces opposedto each other in the width direction of the ridge portion 21, as shownin FIG. 10D.

In order to manufacture the electro-optic modulation device having sucha structure, the top surface of the electro-optic crystal 1 of the rawmaterial shown in FIG. 10A is first cut or polished as shown in FIG. 10Bto form the ridge portion 21 having a width equal to a predeterminedwidth d or less, for example, 0.1 mm or less.

Then, as shown in FIG. 10C, metal 23 is deposited by evaporation orapplied to the top surface of the electro-optic crystal 1 having theformed ridge portion 21. Subsequently, as shown in FIG. 10D, only themetal 23 deposited on the ridge portion 21 by evaporation is removed bypolishing or the like. As a result, the pair of electrodes 25 a and 25 bare formed with metal left on both side faces of the ridge portion 21.

Even if the electro-optic modulation device having such a configurationis formed so as to have an extremely small distance d between the pairof electrodes 25 a and 25 b, the electrodes 25 a and 25 b are formed onthe projected portion of the electro-optic crystal 1 which is large as awhole. Therefore, the electro-optic crystal between the electrodes 25 aand 25 b is not easily broken. In addition, the electrodes 25 a and 25 bare formed by conducting working and metal evaporation on the topsurface of the electro-optic crystal 1. Therefore, it is not difficultto conduct working so as to make the distance d between the electrodes25 a and 25 b located across the ridge portion 21 extremely thin, forexample, 0.1 mm or less.

As shown in FIG. 10D, a spot beam 123 is incident from an end face ofthe electro-optic crystal 1 between the electrodes 25 a and 25 b. As forantireflection coating on the plane of incidence as well, it can beconducted extremely easily and certainly by applying the antireflectioncoating to not only the end face of the thin crystal portion between theelectrodes 25 a and 25 b, but also on an end face of the wholeelectro-optic modulation device including the end face of theelectro-optic crystal 1 formed as one body below the portion. By theway, in the electro-optic modulation device having such a configuration,the direction of an electric-field vector generated by the electrodes 25a and 25 b is perpendicular to the opposed planes of the electrodes 25 aand 25 b.

As shown in FIG. 10D, only the metal 23 deposited on the ridge portion21 by evaporation is removed by polishing or the like, and the pair ofelectrodes 25 a and 25 b are formed of the metal left on both side facesof the ridge portion 21. In this case, the metal 23 remains on the topsurface of the electro-optic crystal 1 besides the side faces opposed toeach other across the ridge portion 23, and this portion also acts aselectrodes. However, undesired electric fields generated betweenelectrodes of this portion are extremely few, and a large majority isgenerated between the opposed electrodes 25 a and 25 b on the ridgeportion 21.

If metal on the remaining portion is removed as shown in, for example,FIG. 11 in order to remove slight or unnecessary electric fieldsgenerated between electrodes formed on the portion of remaining metal,generation of such unnecessary electric fields can be avoided. On theother hand, if the metal of that portion is not removed daringly, anadvantage that the mechanical strength is conversely increased isobtained.

FIGS. 12A to 12D are sectional views showing a manufacture process of anelectro-optic modulation device according to another embodiment of thepresent invention.

The electro-optic modulation device according to the embodiment shown inFIGS. 12A to 12D finally includes a ridge portion 21 a formed on one endof the top surface of the electro-optic crystal 1 so as to project witha predetermined width d or less, for example, with 0.1 mm or less, and apair of electrodes 29 a and 29 b formed on a pair of side faces opposedto each other in the width direction of the ridge portion 21 a, as shownin FIG. 12D.

In order to manufacture the electro-optic modulation device having sucha structure, the top surface of the electro-optic crystal 1 of the rawmaterial shown in FIG. 12A is first cut or polished as shown in FIG. 12Bto form the ridge portion 21 a having a width equal to a predeterminedwidth d or less, for example, 0.1 mm or less.

Then, as shown in FIG. 12C, metal 27 is deposited by evaporation orapplied to the top surface of the electro-optic crystal 1 having theformed ridge portion 21 and a side face on which the ridge portion 21 ais formed inclusive of the ridge portion 21 a. Subsequently, as shown inFIG. 12D, only the metal 27 deposited on the ridge portion 21 a byevaporation is removed by polishing or the like. As a result, the pairof electrodes 29 a and 29 b are formed with metal left on both sidefaces of the ridge portion 21 a.

Even if the electro-optic modulation device having such a configurationis formed so as to have an extremely small distance between the pair ofelectrodes 29 a and 29 b, the electrodes 29 a and 29 b are formed on theprojected portion of the electro-optic crystal 1 which is large as awhole in the same way as the embodiment shown in FIG. 10D. Therefore,the electro-optic crystal between the electrodes 29 a and 29 b is noteasily broken. In addition, the electrodes 29 a and 29 b are formed byconducting working and metal evaporation on the top surface of theelectro-optic crystal 1. Therefore, it is not difficult to conductworking so as to make the distance d between the electrodes 29 a and 29b located across the ridge portion 21 a extremely thin, for example, 0.1mm or less.

As shown in FIG. 12D, a spot beam 123 is incident from an end face ofthe electro-optic crystal 1 between the electrodes 25 a and 25 b. As forantireflection coating on the plane of incidence as well, it can beconducted extremely easily and certainly by applying the antireflectioncoating to not only the end face of the thin crystal portion between theelectrodes 29 a and 29 b, but also on an end face of the wholeelectro-optic modulation device including the end face of theelectro-optic crystal 1 formed as one body below the portion. By theway, in the electro-optic modulation device having such a configuration,the direction of an electric-field vector generated by the electrodes 29a and 29 b is perpendicular to the opposed planes of the electrodes 29 aand 29 b.

As shown in FIG. 12D, only the metal 27 deposited on the ridge portion21 a by evaporation is removed by polishing or the like, and the pair ofelectrodes 29 a and 29 b are formed of the metal left on both side facesof the ridge portion 21 a. In this case, the metal 27 remains on the topsurface of the electro-optic crystal 1 and on side faces besides theside faces opposed to each other across the ridge portion 21 a and thisportion also acts as electrodes. However, undesired electric fieldsgenerated between electrodes of this portion are extremely few, and alarge majority is generated between the opposed electrodes 29 a and 29 bon the ridge portion 21 a.

If in this case as well metal 27 remaining on the top surface of theelectro-optic crystal 1 and on side faces besides the side faces opposedto each other across the ridge portion 21 a is removed, generation ofunnecessary electric fields can be avoided in the same way as the caseshown in FIG. 11.

If it is attempted to increase the length L to obtain a large phasemodulation depth and a large electric-field sensitivity in theembodiments shown in FIGS. 10D, 11 and 12D, the light diffraction effectbecomes an obstacle. In other words, when L is small, light is emittedfrom the end face of the electro-optic crystal even if it is itdiffracted and consequently there is no light loss. When L is madelarge, diffracted light proceeds in such a direction as to get out ofthe ridge portion 21 (21 a).

In the electro-optic modulation devices shown in FIGS. 10D, 11 and 12D,the top surface of the ridge portion 21 (21 a) is in contact with airand both side faces are in contact with the electrodes. At these faces,therefore, reflection takes place and consequently light does not leak.

Since the electro-optic crystal 1 which is the same as the ridge portion21 (21 a) is present under the ridge portion 21 (21 a), however, lightleakage from the ridge portion 21 (21 a) occurs. If the length of theelectro-optic crystal is lengthened, therefore, a large phase modulationdepth and a large electric-field sensitivity corresponding to the lengthcannot be obtained.

Hereafter, an embodiment that provides a large phase modulation depthand a large electric-field sensitivity corresponding to the length evenwhen the length of the electro-optic crystal is lengthened in theelectro-optic modulation devices shown in FIGS. 10D, 11 and 12D will bedescribed.

FIG. 13 is a diagram showing a plane of light incidence of anelectro-optic modulation device according to an embodiment of ridgetype.

The electro-optic modulation device according to this embodimentincludes electro-optic crystal 61 changed in birefringence index byelectric-field coupling, and a low refractive index medium 62 having arefractive index that is less than the refractive index of theelectro-optic crystal 61. It is desirable that the refractive index ofthe low refractive index medium 62 is lower than that of theelectro-optic crystal 61 by at least approximately 10%. For example, ifthe refractive index of the electro-optic crystal 61 is 3, therefractive index of the low refractive index medium 62 should be 2.7 orless. In general, the larger the difference in refractive index betweenthe electro-optic crystal 61 and the low refractive index medium 62becomes, the more desirable. The electro-optic crystal 61 is formed of,for example, GaAs (gallium arsenide), InP (indium phosphide), CdTe(cadmium telluride) or ZnTe (zinc telluride).

Furthermore, the electro-optic modulation device according to theembodiment includes a base portion 63, a ridge portion 64 formed thinly(for example, so as to have a thickness of approximately d=0.1 mm) on atop surface 63 a of the base portion 63 so as to include at least theelectro-optic crystal 61 and have a top surface 61 a exposed to the openair (such as the air), and electrodes 65 a and 65 b each having anL-shaped section that extends over opposed side faces 64 a and 64 b ofthe ridge portion 64 and the top surface 63 a of the base portion 63.The electro-optic crystal 61 is sandwiched between the open air abovethe top surface 61 a and the low refractive index medium 62.

In the electro-optic modulation device according to this embodiment, theL-shaped electrodes 65 a and 65 b are provided so as to extend over theside faces 64 a and 64 b and the top surface 63 a of the base portion63. As compared with the case where the electrodes 65 a and 65 b areprovided respectively on the side faces 64 a and 64 b, therefore, themechanical strength is improved. For example, the possibility of thebase portion 63 and the ridge portion 64 being separated from each otheror a part of the ridge portion 64 being damaged can be reduced.

If light incident from a beam spot BS of the electro-optic crystal 61 isdiffracted, for example, upward (in the positive y-direction) in theelectro-optic modulation device according to the embodiment, the lightis reflected by the open air above the top surface 61 a and returnedinto the electro-optic crystal 61. If light is diffracted, for example,downward (in the negative y-direction), the light is reflected by thelow refractive index medium 62 and returned into the electro-opticcrystal 61. At the electrodes 65 a and 65 b as well, reflection takesplace in the same way. In other words, an optical waveguide isconstructed in the electro-optic modulation device according to theembodiment. At this time, the electro-optic crystal 61 is equivalent toa core in the optical waveguide, and the low refractive index medium 62is equivalent to a clad in the optical waveguide. Thus, in theelectro-optic modulation device according to the embodiment, light canbe trapped in the electro-optic crystal 61. Even if the length of theelectro-optic crystal 61 in the z-direction is lengthened, therefore, itis possible to prevent diffracted light from being leaked. As a result,a large phase modulation depth and a large electric-field sensitivitycan be obtained.

Especially, in an example shown on the left side in FIG. 13, the baseportion 63 is formed of the low refractive index medium 62 and the ridgeportion 64 is formed of the electro-optic crystal 61. As compared withan example shown in the center and an example shown on the right sidedescribed later, therefore, the structure can be simplified andconsequently the manufacture of the electro-optic modulation device isfacilitated. For example, it is facilitated to manufacture the baseportion 63 and the ridge portion 64 separately and couple them later.Unlike the example shown in the center and described later, a projectionis not formed in the low refractive index medium 62 and the possibilityof damage in the base portion 63 and the ridge portion 64 can be madelow.

In the example shown in the center of FIG. 13, a lower part of the ridgeportion 64 is formed of the low refractive index medium 62. If therefractive index of the low refractive index medium 62 is not so smallas compared with the refractive index of the electro-optic crystal 61,oozing out of light into the low refractive index medium 62 becomescomparatively large. In this example, the electric field is coupled tothe light that has oozed out, as well. If the low refractive indexmedium 62 forming the lower part of the ridge portion 64 has anelectro-optic effect, therefore, the detection sensitivity can be madehigh. Furthermore, unlike the left side example and the right sideexample in which the electrodes 65 a and 65 b do not face the light thathas oozed out, the sensitivity is not lowered in this example.

In the example shown on the right side in FIG. 13, an upper part of thebase portion 63 is formed of the electro-optic crystal 61, andconsequently the electro-optic crystal 61 becomes large. In particular,the projection area from the upward becomes large. As compared with theleft side example and the center example, therefore, the mechanicalstrength of the electro-optic modulation device can be increased. Forimproving the sensitivity, the ridge portion is formed as thin aspossible. In the right side example, the electro-optic crystal 61forming the ridge portion 64 becomes large as a whole and consequentlyit becomes easy to handle the ridge portion 64. For example, therefore,the work of conducting antireflection coating on the end face of theelectro-optic crystal 61 is facilitated.

By the way, it is also possible to use photonic crystal having aperiodic structure instead of the low refractive index medium 62according to the embodiment. The photonic crystal is a generic term ofmaterials having a periodic structure of a light wavelength order. Thephotonic crystal has a property of preventing light from entering aregion having a periodic structure.

When using a medium formed of photonic crystal 71 as shown in FIG. 14A,it is possible to construct an electro-optic modulation device includinga ridge portion 71 a formed of a region having no periodic structure anda base portion 71 b formed of a region having a periodic structure byconducting cutting working on electro-optic crystal including a regionhaving no periodic structure and a region having a periodic structure.

An electro-optic modulation device may be constructed by adheringelectro-optic crystal 73 and photonic crystal 75 having a periodicstructure to each other with an adhesive agent 77 as shown in FIG. 14Band then cutting the electro-optic crystal 73.

FIG. 15 is a diagram showing a plane of light incidence of anelectro-optic modulation device according to another embodiment of ridgetype.

In the electro-optic modulation device according to this embodiment,kinds of chemical elements included in the low refractive index mediumin the embodiment described earlier are made the same as kinds ofchemical elements included in the electro-optic crystal, and refractiveindexes are made different from each other according to a difference incomposition ratio of the chemical elements. Other configurations anddifferences among examples, operation and effects are not different fromthose of the electro-optic modulation device in the above-describedembodiment, and consequently description of them will be omitted.

In the electro-optic modulation device according to this embodiment,kinds of chemical elements included in the low refractive index mediumin the embodiment described earlier are made the same as kinds ofchemical elements included in the electro-optic crystal. By onlychanging the composition ratio of the chemical elements after formingthe low refractive index medium in the crystal growth process,therefore, the electro-optic crystal can be formed continuously. As aresult, integral electro-optic crystal 61A including a high refractiveindex layer and a low refractive index layer is obtained. As comparedwith the case where the electro-optic crystal and the low refractiveindex medium are manufactured separately and coupled, manufacture isfabricated. Furthermore, thickness adjustment of the low refractiveindex medium and the electro-optic crystal can be conducted easily.Furthermore, since a boundary plane between the low refractive indexmedium and the electro-optic crystal can be made similar to an idealplane, light leak can be reduced as compared with the case where thereare a large number of concavities and convexities on this boundaryplane.

FIG. 16 is a diagram showing a plane of light incidence of anelectro-optic modulation device according to another embodiment of ridgetype.

The electro-optic modulation device according to this embodimentincludes electro-optic crystal 61, and an adhesive agent 62 a serving asa low refractive index medium having a refractive index that is lessthan the refractive index of the electro-optic crystal 61.

The electro-optic modulation device according to this embodimentincludes a base portion 63, a ridge portion 64 formed thinly on a topsurface 63 a of the base portion 63 so as to include at least theelectro-optic crystal 61 and have a top surface 61 a exposed to the openair, and L-shaped electrodes 65 a and 65 b each of which extends overopposed side faces 64 a and 64 b of the ridge portion 64 and the topsurface 63 a of the base portion 63. The electro-optic crystal 61 isformed to be sandwiched between the open air above the top surface 61 aand the adhesive agent 62 a.

In the electro-optic modulation device according to this embodiment aswell, the L-shaped electrodes 65 a and 65 b are provided so as to extendover the side faces 64 a and 64 b and the top surface 63 a of the baseportion 63. As compared with the case where the electrodes 65 a and 65 bare provided respectively only on the side faces 64 a and 64 b,therefore, the mechanical strength is improved.

In the electro-optic modulation device according to this embodiment aswell, an optical waveguide is constructed, and consequently light can betrapped in the electro-optic crystal 61. By lengthening the length ofthe electro-optic crystal 61, therefore, it becomes possible to obtain alarge phase modulation depth and a large electric-field sensitivitycorresponding to the length.

Furthermore, the substrate 66 a and the electro-optic crystal 61 can becoupled by using the adhesive agent 62 a.

In an example shown on the left side in FIG. 16, the base portion 63 isformed of the substrate 66 and the adhesive agent 62 a disposed abovethe substrate 66, and the ridge portion 64 is formed of theelectro-optic crystal 61. As a result, the area of contact with theelectrodes 65 a and 65 b becomes wide. Therefore, the electrodes 65 aand 65 b can be fixed firmly. Furthermore, it becomes unnecessary to useanother adhesive agent to fix the electrodes 65 a and 65 b.

In the example shown in the center of FIG. 16, the base portion 63 isformed of the substrate 66, and a lower part of the ridge portion 64 isformed of the adhesive agent 62 a. Therefore, operation and effectssimilar to those in the example shown in the center in anotherembodiment are obtained.

In the example shown on the right side in FIG. 16, the base portion 63is formed of the substrate 66, the adhesive agent 62 a disposed abovethe substrate 66, and the electro-optic crystal 61 disposed above theadhesive agent 62 a, and the ridge portion 64 is formed of theelectro-optic crystal 61. Therefore, operation and effects similar tothose in the example shown in the right side in another embodiment areobtained.

In the above-described embodiments, the low refractive index medium 62or the adhesive agent 62 a are provided on the bottom surface of theelectro-optic crystal 61 which is one of surfaces that extend along thepath of light, and the top surface 61 a is exposed to the open air.Alternatively, the electro-optic crystal 61 may be sandwiched betweenlow refractive index media by providing a low refractive medium on thetop surface 61 a as well.

FIGS. 17A and 17B are diagrams showing a manufacture method of anelectro-optic modulation device according to another embodiment of ridgetype.

In this embodiment, an electro-optic modulation device having a cavity81 a under a ridge portion 81 b may be constructed by, for example,cutting electro-optic crystal 81 having the cavity 81 a formed by thecrystal growth process as shown in FIG. 17A.

Furthermore, as shown in FIG. 17B, an electro-optic modulation devicehaving a cavity 89 under a ridge portion may be constructed by adheringelectro-optic crystal 83 having the ridge portion formed thereinpreviously and a base portion 85 having a hollow formed in its topportion to each other by means of an adhesive agent 87.

As gas having a refractive index lower than the refractive index of theelectro-optic crystal, for example, air or gas can be sealed in thesecavities 81 a and 89. It is possible to cause the open air to flow intoand out of these cavities 81 a and 89. These cavities 81 a and 89 can bemade vacuous.

In the above-described embodiment, an example premised on the case shownin FIG. 10D has been mentioned. As a matter of course, the embodimentcan be applied to the case shown in FIG. 11. In addition, the embodimentcan be applied to the case shown in FIG. 12D as well in the same way.Since they are self-evident to those skilled in the art, description ofconcrete examples with reference to drawings will be omitted.

An embodiment of an electro-optic modulation device relating to aconfiguration in which the frequency characteristics become flat willnow be described.

In an electro-optic modulation device including electro-optic crystaland a pair of electrodes with the electro-optic crystal sandwichedtherebetween, the electro-optic crystal is distorted mainly in adirection perpendicular to the electrode plane, and consequentlyflatness in frequency characteristics can not be obtained. Hereafter,therefore, several embodiments in which the distortion of theelectro-optic crystal is reduced by wax or the like will be described.

FIGS. 18A to 18E are diagrams showing how wax is applied toelectro-optic crystal placed longitudinally on a pedestal 19.

FIG. 18A shows the case where wax 37 is applied so as to heap bothelectrodes 33 and 35 with the wax 37 from a top surface of theelectro-optic crystal 31 and in addition heap the pedestal 19 as wellwith the wax 37. According to this aspect, the distortion of theelectro-optic crystal 31 can be suppressed certainly. FIG. 18B shows thecase where wax 37 is applied so as to heap one electrode 33 with the wax37 from a top surface of the electro-optic crystal 31 and in additionheap the pedestal 19 as well with the wax 37. In this case as well, thedistortion of the electro-optic crystal 31 can be suppressedcomparatively certainly. As a matter of course, the electrode 35 sidemay be heaped with the wax 37. FIG. 18C shows the case where wax 37 isapplied so as to heap a pedestal 19 with the wax from both electrodes 33and 35. By thus fixing the electrodes 33 and 35 to the pedestal 19 bymeans of the wax 37, distortion of the electro-optic crystal 31 can besuppressed relatively. FIG. 18D shows the case where wax 37 is appliedso as to heap the top surface of the electro-optic crystal 31 with thewax 37. If the electrodes 33 and 35 are thus fixed to the electro-opticcrystal 31, distortion of the electro-optic crystal 31 can besuppressed. FIG. 18E shows the case where wax 37 is applied so as toheap the top surface of the electro-optic crystal 31 and top endportions of the both electrodes 33 and 35 with the wax 37. In this caseas well, distortion of the electro-optic crystal 31 can be suppressed inthe same way as the case shown in FIG. 18D. By the way, wax is notapplied to a beam spot BS or the surface of the beam spot. This aims atpreventing an optical beam from being diffracted by wax.

Which of the above-described aspects is selected depends upon theviewpoint of tradeoff between the degree of the frequency flatness andthe degree of the electro-optic effect. In other words, in the aspectshown in FIG. 18A, distortion of the electro-optic crystal 31 can besuppressed certainly, but there is a drawback that the electro-opticeffect falls somewhat. On the other hand, in the aspect shown in FIG.18D, distortion of the electro-optic crystal 31 cannot be suppressed somuch as compared with the aspect shown in FIG. 18A, but theelectro-optic effect cannot fall so much.

FIG. 19 is a diagram showing differences in output characteristics of anelectric-field sensor among the case where wax is not applied to theelectro-optic crystal, the case where the top surface of theelectro-optic crystal 31 is heaped with wax 37 as shown in FIG. 18D, andthe case where both electrodes 33 and 35 and further the pedestal 19 areheaped with wax 37 from the top surface of the electro-optic crystal 31.

In the electric-field sensor having the configuration shown in FIG. 1,it is desirable that the amplitude voltage (output amplitude voltage) ofthe output signal 122 is flat. In the case where electro-optic crystal31 of a certain kind is used and the wax 37 is not applied to theelectro-optic crystal 31, however, resonance is found near 590 kHz, near610 kHz and near 720 kHz. If the wax 37 is applied to the top surface ofthe electro-optic crystal 31 as shown in FIG. 18D, however, resonancecan be reduced while maintaining the output amplitude voltage.Furthermore, if the wax 37 is applied to the top surface of theelectro-optic crystal 31 and both the electrodes 33 and 35 as shown inFIG. 18A, resonance can be eliminated although the output amplitudevoltage becomes low.

FIGS. 20A to 20E are diagrams showing how wax is applied toelectro-optic crystal placed laterally on the pedestal 19.

FIG. 20A shows the case where the wax 37 is applied so as to heap bothside faces of the electro-optic crystal 31 with the wax 37 from anelectrode 33 disposed on the electro-optic crystal 31 and further heapthe pedestal 19 as well with the wax 37. According to this aspect, thedistortion of the electro-optic crystal 31 can be suppressed certainly.FIG. 20B shows the case where wax 37 is applied so as to heap one of theside faces of the electro-optic crystal 31 with the wax 37 from a topsurface of the electrode 33 and in addition heap the pedestal 19 as wellwith the wax 37. In this case as well, the distortion of theelectro-optic crystal 31 can be suppressed comparatively certainly. As amatter of course, the other side face of the electro-optic crystal 31may be heaped with the wax 37. FIG. 20C shows the case where wax 37 isapplied so as to heap the pedestal 19 with the wax from both side facesof the electro-optic crystal 31. In this case, the electro-optic crystal31 can be fixed to both the electrodes 33 and 35, and can be furtherfixed to the pedestal 19 as well. Therefore, distortion of theelectro-optic crystal 31 can be suppressed comparatively certainly. FIG.20D shows the case where wax 37 is applied so as to heap both side facesof the electro-optic crystal 31 with wax 37 from an end portion of theelectrode 33 and heap the pedestal 19 with the wax 37 from both sidefaces of the electro-optic crystal 31. In this case as well, the effectcan be obtained in the same way as the case shown in FIG. 20C. FIG. 20Eshows the case where wax 37 is applied so as to heap both side faces ofthe electro-optic crystal 31 with the wax 37. Since the electro-opticcrystal 31 is fixed to both electrodes 33 and 35 by the wax 37,distortion of the electro-optic crystal 31 can be suppressed. By theway, wax is not applied to a beam spot BS or the surface of the beamspot. This aims at preventing an optical beam from being diffracted bywax.

In the description of FIGS. 18A to 18E and 20A to 20E, wax is applied.However, the applied material is not restricted to wax, but anotherinsulator may be used.

FIGS. 21A and 21B are diagrams showing how wax is applied to anelectro-optic modulation device of the above-described so-called H-type.

In an aspect shown in FIG. 21A, the insulators 9 a and 9 b in theelectro-optic modulation device of H-type shown in FIGS. 5A and 5B arespecifically replaced by wax 10 a and 10 b. In other words, electrodes 7a and 7 b are formed with a central thin crystal portion sandwichedtherebetween in grooves 3 a and 3 b of the electro-optic crystal 1placed on the pedestal 19 in the same way as the embodiment shown inFIGS. 5A and 5B. Unlike the embodiment shown in FIGS. 5A and 5B,however, wax 10 a and 10 b are embedded in remaining groove portions asa concrete example of the insulators 9 a and 9 b.

According to such the aspect, the central thin crystal portion iscompletely surrounded and fixed by the electrodes 7 a and 7 b, wax 10 aand 10 b, and other electro-optic crystal. Therefore, distortion of thethin crystal portion sandwiched between the electrodes 7 a and 7 b canbe suppressed. By the way, the aspect shown in FIG. 21 further has aneffect of complementing the physical strength of the central thincrystal portion in the same way as the aspect shown in FIGS. 5A and 5B.

In an aspect shown in FIG. 21B, the whole of the electro-opticmodulation device of H-type inclusive of the grooves 3 a and 3 b of theelectro-optic crystal 1 is covered by wax 10, and the electro-opticmodulation device is fixed to the pedestal 19 by the covering wax 10.According to such an aspect as well, it is a matter of course that thedistortion of the central thin crystal portion can be suppressed and itsphysical strength can be complemented. The applied material is notrestricted to the wax 10, but another insulator may be used.

FIGS. 22A to 22C are diagrams showing how wax is applied to anelectro-optic modulation device of the so-called ridge type.

In an aspect shown in FIG. 22A, wax 10 is applied to a top surface ofthe ridge portion 21 including electrodes of the electro-opticmodulation device of ridge type shown in FIG. 10D. According to such anaspect, the ridge portion 21 and the electrodes 25 a and 25 b can befixed. Therefore, the distortion of the crystal in the ridge portion 21can be suppressed.

In an aspect shown in FIG. 22B, the whole of the ridge portion 21 andthe electrodes 25 a and 25 b of the electro-optic modulation device ofridge type shown in FIG. 10D is covered by wax 10. According to such anaspect as well, the ridge portion 21 and the electrodes 25 a and 25 bcan be fixed. Therefore, the distortion of the crystal in the ridgeportion 21 can be suppressed.

Furthermore, in the aspect shown in FIG. 22B, the whole of theelectro-optic modulation device of ridge type shown in FIG. 10D iscovered by wax 10 and fixed to the pedestal 19. It is a matter of coursethat in this case well distortion of the crystal in the ridge portion 21can be suppressed.

Which of the aspects shown in FIGS. 22A to 22C is selected depends uponthe viewpoint of tradeoff between the degree of the frequency flatnessand the degree of the electro-optic effect in the same way as the casesshown in FIGS. 18A to 18E.

In the description of FIGS. 22A to 22E, the ridge portion and so on iscovered by wax. However, the ridge portion and so on may be covered byanother insulator.

It is a matter of course that the aspects shown in FIGS. 22A to 22C canalso be applied to the electro-optic modulation device of the so-calledL-type shown in FIG. 12D.

In the above-described embodiments, a matter having viscosity and theproperty of becoming hard with the lapse of time is applied to theelectro-optic crystal, as heretofore described. Therefore, the matterthat has become hard reduces deformation of the crystal lattice. As aresult, an electro-optic modulation device free from resonance andhaving flatter frequency characteristics is obtained.

In the present embodiment, the matter having viscosity and the propertyof becoming hard with the lapse of time is used. If it is a matter thathas viscosity of such a degree that at least its shape is not changedwhen applied to electro-optic crystal, it is suitable because its shapeis maintained even if the matter is applied so as to take a desiredshape and then left as it is.

Wax becomes hard by evaporation of moisture with elapse of time.Alternatively, a matter that becomes low in temperature and consequentlybecomes hard with the lapse of time, i.e., a matter preheated so as tohave viscosity may be used. An adhesive agent may be used.

INDUSTRIAL APPLICABILITY

According to the present invention, the electro-optic crystal includesgrooves formed respectively on one pair of side faces that are parallelto a direction of light incident between a pair of electrodes, so as tobecome parallel to the direction, and consequently a thin crystalportion sandwiched between the grooves serves as a portion for couplingthe electric field. The grooves are filled with one pair of electrodes,or filled with one pair of electrodes and insulators. Therefore, theelectro-optic crystal is not easily broken from the thin crystal portionbetween the electrodes. In addition, it is not difficult to work theelectro-optic crystal between the electrodes so as to make it extremelythin. In addition, antireflection coating can be conducted extremelyeasily and certainly by applying the antireflection coating to not onlyan end face of the thin crystal portion between the electrodes, but alsoon an end face of the whole electro-optic modulation device includingthe end face of the electro-optic crystal except the crystal portion.This results in an effect that the thin crystal portion is not distortedand the frequency characteristics become flat.

According to the present invention, a ridge portion having a widthshorter than a predetermined width projected on one side face of a baseportion is formed as electro-optic crystal coupled to electric field.Therefore, a thin crystal portion between the electrodes is not easilybroken. In addition, it is not difficult to conduct working so as tomake the ridge portion between the electrodes extremely thin, forexample, 0.1 mm or less. As for antireflection coating on the plane ofincidence as well, it can be conducted extremely easily and certainly bygenerally applying the antireflection coating to not only the end faceof the thin crystal portion between the electrodes, but also on an endface of the whole electro-optic modulation device including the end faceof the electro-optic crystal integrally formed under the crystalportion.

If in this case at least the top surface of the ridge portion and sidefaces of one pair of electrodes forming faces continuous to the topsurface are covered by an insulator, distortion of the electro-opticcrystal in the ridge portion is suppressed and flat frequencycharacteristics are obtained.

Furthermore, if in this case at least the refractive index of a boundaryportion between the ridge portion and the base portion is made lowerthan the refractive index of the electro-optic crystal in the ridgeportion, it is possible to prevent diffracted light from leaking evenwhen the length of the electro-optic crystal is lengthened. Therefore, alarge phase modulation depth can be obtained.

Furthermore, according to the present invention, an insulator is appliedso as to relatively fix the electro-optic crystal and one pair ofelectrodes. Therefore, distortion of the electro-optic crystal issuppressed and flat frequency characteristics are obtained.

1. An electro-optic modulation device that includes an electro-opticcrystal having a birefringence index changed by a coupled electricfield, and one pair of electrodes disposed so as to have theelectro-optic crystal interposed therebetween to couple the electricfield to the electro-optic crystal, and that changes polarization oflight incident between the one pair or electrodes according to a changeof the birefringence index depending upon a strength of electric fieldcoupled via the one pair of electrodes, the electro-optic modulationdevice comprising: a base portion having a top surface; a ridge portionprojecting from the top surface and extending in a direction of theincident light, at least a part of the ridge portion comprising theelectro-optic crystal, the ridge portion having a width equivalent to apredetermined value or less; and an insulator which covers the wholedevice, wherein the electrodes are formed on one pair of side facesopposed in a width direction of the ridge portion and on the whole topsurface adjacent to the side faces.
 2. An electro-optic modulationdevice that includes an electro-optic crystal having a birefringenceindex changed by a coupled electric field, and one pair of electrodesdisposed so as to have the electro-optic crystal interposed therebetweento couple the electric field to the electro-optic crystal, and thatchanges polarization of light incident between the one pair ofelectrodes according to a change of the birefringence index dependingupon a strength of electric field coupled via the one pair ofelectrodes, the electro-optic modulation device comprising: a baseportion having a top surface; and a ridge portion projecting from thetop surface and extending in a direction of the incident light, at leasta part of the ridge portion comprising the electro-optic crystal, theridge portion having a width equivalent to a predetermined value orless, wherein the electrodes are formed on one pair of side facesopposed in a width direction of the ridge portion and on the whole topsurface adjacent to the side faces, and an insulator covers the ridgeportion and at least parts of the electrodes, formed on the top surface.3. An electro-optic modulation device that includes an electro-opticcrystal having a birefringence index changed by a coupled electricfield, and one pair of electrodes disposed so as to have theelectro-optic crystal interposed therebetween to couple the electricfield to the electro-optic crystal, and that changes polarization oflight incident between the one pair of electrodes according to a changeof the birefringence index depending upon a strength of electric fieldcoupled via the one pair of electrodes, the electro-optic modulationdevice comprising: a base portion having a top surface; and a ridgeportion projecting from the top surface and extending in a direction ofthe incident light, at least a part of the ridge portion comprising theelectro-optic crystal, the ridge portion having a width equivalent to apredetermined value or less, wherein the electrodes are formed on onepair of side faces opposed in a width direction of the ridge portion andon the whole top surface adjacent to the side faces, and an insulatorcovers a top surface of the ridge portion and side faces of the one pairof electrodes which are continuous with the top surface of the ridgeportion.
 4. The electro-optic modulation device according to claim 1,wherein the insulator comprises wax.